1. Field of the Invention
The present invention relates to an illumination apparatus for illuminating an image display plane of a liquid crystal display device, and more specifically, relates to a scheme intended to realize a uniform in-plane brightness of the illumination apparatus in a method for converting a point light source into a plane light source. By means of the present invention, an illumination apparatus capable of emitting light as a plane light source with no unevenness in brightness can be realized even when a point light source is employed. In addition, by means of the present invention, an illumination apparatus capable of emitting light as a plane light source with less unevenness in brightness can be realized even when the small number of point light sources is employed.
2. Description of the Related Art
A liquid crystal electro-optical device is widely used in view of advantages of low power consumption, light weight, and a small thickness. The liquid crystal electro-optical device includes a direct-view type liquid crystal electro-optical device and a projection-type liquid crystal electro-optical device. In the case of a direct-view and transmission type liquid crystal electro-optical device, a viewer recognizes an image by means of a back light. In the case of a direct-view and reflection type liquid crystal electro-optical device, a viewer recognizes an image by means of a front light.
FIG. 22 shows a perspective view of an edge-light type back light in which light sources are disposed at side surfaces of a plate-like light guiding plate. More specifically, the light sources 104, each of which is a line light source such as a cold cathode fluorescent tube or the like, are provided at two opposite side surfaces of the plate-like light guiding plate 105. Light incident onto the plate-like light guiding plate 105 is scattered by means of ink dots 106 formed on a rear surface of the plate-like light guiding plane to emit toward a transmission type liquid crystal electro-optical device 101. A prism sheet 103 may be used over the plate-like light guiding plate in order to enhance brightness in the front direction. Light emitted from the plate-like light guiding plate and provided with directionality by means of the prism sheet is incident on a diffusion plate 102 so that the in-plane brightness distribution can to become uniform by means of the diffusion plate. Light scattered by the ink dots and leaked downward from the plate-like light guiding plate is reflected by reflecting plate 107 to travel back toward the liquid crystal electro-optical device 101.
Thus, the illumination apparatus such as a back light is provided with a plate-like light guiding plate disposed below a display region of a liquid crystal electro-optical device, and further provided with line light sources disposed at the side surfaces of the plate-like light guiding plate. Light emitted from the light sources repeats total reflections within the plate-like light guiding plate to be expanded over the entire region of the plate-like light guiding plate. FIGS. 20A and 20B respectively show cross-sectional views of the plate-like light guiding plate in the thickness direction thereof, illustrating different manners of light propagation in the plate-like light guiding plate. It should be noted that six surfaces are defined for the plate-shaped light guiding plate as shown in a perspective view of FIG. 19A, in order to explain the light propagation. More specifically, a surface closer to a viewer is referred to as an upper surface 735, while a surface opposite to the upper surface is referred to as a lower surface 736. A side surface onto which a light emitted from a light source 737 is incident is referred to as an end surface 738. Each of surfaces perpendicular to the end surface is referred to as a side surface 739. The last surface is a surface 740, which is parallel to the end surface.
FIG. 20A illustrates the light propagation in the case where light is incident along the end surface 109 of the plate-like light guiding plate having the refractive index of 1.49 from the air 112 having the refractive index of 1. The light incident along the end surface of the plate-like light guiding plate is refracted in accordance with the Snell's law to be propagated at the angle of 42° with respect to the normal direction of the end surface of the plate-like light guiding plate, and is then incident on the lower surface 110 of the plate-like light guiding plate at the angle of 48° which exceeds the critical angle, thereby being totally reflected. Thereafter, the light is incident on the upper surface 111 of the plate-like light guiding plate at the angle of 48° to be totally reflected. Thus, the light repeats the total reflections at the upper surface 111 of the plate-like light guiding plate and the lower surface of the plate-like light guiding plate. FIG. 20B illustrates the light propagation in the case where light is incident at the angle (θ1) smaller than 90° with respect to the normal direction of the end surface 109 of the plate-like light guiding plate 105 having the refractive index of 1.49 from the air having the refractive index of 1. The light entering the plate-like light guiding plate is incident on the upper surface 111 of the plate-like light guiding plate and the lower surface 110 of the plate-like light guiding plate at the angle (θ2), which exceeds the critical angle. Thus, the light repeats the total reflections at the upper surface of the plate-like light guiding plate and the lower surface of the plate-like light guiding plate, thereby resulting in the light being emitted from the surface parallel to the end surface 109 while being inclined at the angle of θ1 with respect to the normal direction of this surface.
Thus, the light incident on the end surface 109 of the plate-like light guiding plate at any angle is entirely totally reflected within the plate-like light guiding plate. Accordingly, no light is allowed to emit through the upper surface of the plate-like light guiding plate or the lower surface of the plate-like light guiding plate, so long as no structural member is provided at the upper or lower surface of the plate-like light guiding plate. In addition, as calculated from the Snell's law, the light incident from the air onto the end surface of the plate-like light guiding plate at any angle is refracted at the interface between the air and the end surface of the plate-like light guiding plate, so that the light propagating within the plate-like light guiding plate is inclined with respect to the normal direction of the end surface of the plate-like light guiding plate at 42° or less.
In the case where it is desired to emit the light through the upper surface of the plate-like light guiding plate, white-colored ink dots may be provided at the lower surface of the plate-like light guiding plate. FIG. 23 illustrates a cross-sectional view of an edge-light type back light. Like reference numerals designate like components both in FIGS. 22 and 23. A light source 104 is provided in the vicinity of an end surface 109 of the plate-like light guiding plate, and a lamp reflector 108 is formed around the light source. Light emitted from the light source and light reflected from the lamp reflector are allowed to enter a plate-like light guiding plate through the end surface of the plate-like light guiding plate 105. The light is incident on the upper surface 111 of the plate-like light guiding plate and the lower surface 110 of the plate-like light guiding plate to be totally reflected within the plate-like light guiding plate. However, since the white-colored ink dots 106 are printed on the lower surface of the plate-like light guiding plate, the light incident onto the ink dots 106 is scattered due to the shape or the refractive index of the ink dots. When the light is thus scattered by the ink dot and is allowed to be incident on the upper surface 111 of the plate-like light guiding plate at the angle smaller than the critical angle, the light is allowed to exit from the plate-like light guiding plate. Thus, by optimizing the size, the pitch and the density of the ink dots, the in-plane brightness of the light exiting the plate-like light guiding plate can be made uniform.
The illumination apparatus in which the light is emitted through the lower surface of the plate-like light guiding plate can be applied to a front light of a reflection type liquid crystal electro-optical device. In the case of the direct-view and reflection type liquid crystal electro-optical device, a display region of the reflection type liquid crystal electro-optical device is irradiated with the illumination from the front light, so that a viewer can recognize an image. The front light is lit under the small amount of external light so that the image can be easily viewed.
FIG. 24A illustrates a cross-sectional view of a prism-type front light as one example of the front light. A plate-like light guiding plate 202 provided with a prism surface on its upper surface is formed over a display region of a reflection type liquid crystal electro-optical device 201. Adjacent to an end surface 213 of the plate-like light guiding plate, a light source 203 is disposed. In order to effectively guide the light emitted from the light source toward the end surface of the plate-like light guiding plate, a lamp reflector 204 is provided. A cross-sectional view in FIG. 24B illustrates an operation of the prism-type front light when the light is off. When the light source is off, external light 205 passes through the plate-like light guiding plate 202 and is then reflected from the reflection type liquid crystal electro-optical device 201, so that the reflected light containing the image information is emitted toward the viewer. On the other hand, a cross-sectional view in FIG. 24C illustrates an operation of the prism-type front light when the light is on. When the light source 203 is on, light 206 emitted from the light source 203 is reflected from the lamp reflector 204 to be incident on the end surface 213 of the plate-like light guiding plate 202. The light 206 incident on the plate-like light guiding plate 202 is then surface-reflected at a side surface of the prism to be incident on the reflection type liquid crystal electro-optical device 201. The light reflected from the reflection type liquid crystal electro-optical device 201 is incident on the interface between the plate-like light guiding plate and the air at the angle smaller than the critical angle, thereby being allowed to exit from the plate-like light guiding plate.
In an alternative embodiment mode of the front light of the reflection type liquid crystal electro-optical device, projections may be provided on the lower surface of the plate-like light guiding plate. FIG. 25A illustrates a cross-sectional view of the projection-shape front light. On a lower surface of a plate-like light guiding plate 207, projections 208 each having a rectangular cross-section are formed. The shape of the to projections is not limited to a rectangular shape, but may be corrugated. In order to effectively guide the light emitted from a light source 209 toward an end surface of the plate-like light guiding plate, a lamp reflector 210 is provided. A reflection type liquid crystal electro-optical device 212 is disposed below the plate-like light guiding plate. A cross-sectional view in FIG. 25B illustrates an operation of the projection-shape front light when the light is off. When the light source is off, the external light 211 passes through the plate-like light guiding plate 207 and is then reflected from the reflection type liquid crystal electro-optical device 212 to be emitted toward the viewer. On the other hand, a cross-sectional view in FIG. 25C illustrates an operation of the projection-shape front light when the light is on. When the light source 209 is on, the light 213 emitted from the light source 209 is reflected from the lamp reflector 210 to be incident on the end surface 207 of the plate-like light guiding plate. When the light incident on the end surface of the plate-like light guiding plate propagates within the plate-like light guiding plate to be incident on a bottom surface of the projection 208 formed on the lower surface of the plate-like light guiding plate, the light is totally reflected so as to propagate within the plate-like light guiding plate. When the light is incident on a side surface of the projection 208, the total-reflection condition of the light is not met so that the light is refracted at the side surface. Most of the thus refracted light is incident on the reflection type liquid crystal electro-optical device, so that the reflected light containing the image information is allowed to emit toward the viewer. Thus, in the projection-shape front light, the total-reflection condition is not met for the light incident on the side surface of the projection provided on the lower surface of the plate-like light guiding plate, so that the light is incident on the reflection type liquid crystal electro-optical device. In order to allow the light to be uniformly incident on the reflection type liquid crystal electro-optical device, the projections are formed at a lower density in the vicinity of the light source while at a higher density as further away from the light source.
Since the liquid crystal electro-optical device is of the non-emission type, the device is used by projecting light thereto from a back light or a front light in order to improve the visibility of a display. As a light source of the back light or the front light, a cold cathode fluorescent tube is generally used. However, when the cold cathode fluorescent tube is used as the light source, most of power consumption of the liquid crystal display device is derived from the back tight or the front light. In order to reduce the power consumption of the liquid crystal display device, a light emitting diode (LED) is recently used as the light source instead of the cold cathode fluorescent tube. Use of the light emitting diode can suppress the power consumption to a fraction of that necessary when the cold cathode fluorescent tube is used.
Since the light emitting diode is a point light source, it can have the size of about 1 mm×1 mm and the thickness of about 2 to 3 mm. In order to reduce the size of the liquid crystal display device, the light emitting diode can be employed. Since the light emitting diode is a point light source, means for converting such a point light source into a plane-like light source having a high uniformity of in-line brightness is required.
In an attempt where a point light source such as a light emitting diode is converted into a plane light source so as to obtain a uniform lightness in a large area, unevenness in the brightness cannot be avoided. In an example for converting the point light source into the plane light source, as shown in a top plan view of FIG. 21 in which a plurality of point light sources 301 to 303 such as a light emitting diode are disposed on a side surface of a plate-like light guiding plate 304, light incident from the point light sources onto the plate-like light guiding plate is expanded in a plane within the plate-like light guiding plate. However, even when a plurality of point light sources are disposed on the side surface of the plate-like light guiding plate, these point light sources can not be converted into a uniform plane light source. As previously explained, when an acrylic resin is used for the plate-like light guiding plate, light is incident from the air having the refractive index of 1 onto the acrylic resin having the refractive index of 1.49, and therefore, refraction occurs due to a difference in refractive indices of the involved materials. As can be calculated from the Snell's law, the light refracted at the interface between the air and the plate-like light guiding plate is expanded only up to the maximum angle (θA) of 42° with respect to the normal direction of the incident surface of the plate-like light guiding plate. Thus, even when the light emitted from the point light sources is incident on the plate-like light guiding plate, the light is expanded only over certain regions of the plate-like light guiding plate while the light is not expanded to some regions 305 therein. In the case where the illumination light is employed as a front light or a back light for a liquid crystal electro-optical device, the brightness on an image area has to be uniform. With a large unevenness in the brightness, the visibility is significantly damaged. Even when a diffusion plate is provided between the point light sources 301 to 303, such as light emitting diodes, and the plate-like light guiding plate 304, uniformity in the diffused light is not satisfactory so that in-plane unevenness in the brightness is induced for the illumination light emitted from the back light or the front light.
An example of an illumination apparatus in which one point light source and a plate-like light guiding plate are employed is described, for example, in Japanese Laid-Open Patent Publication No. 10-199318. In this illumination apparatus, a point light source is disposed at the center portion of a side surface of the plate-like light guiding plate. More specifically, as shown in a plan view of FIG. 31, the illumination apparatus includes only a plate-like light guiding plate 304 and a point light source 307 at the center portion of a side surface of the plate-like light guiding plate, and therefore, the light of the point light source expanded within the plate-like light guiding plate can not spread over the entire display region, so that corner areas 306 of the display region become dark.
Means for converting a point light source into a plane light source is desirably means for obtaining a bright plane light source having a satisfactory uniform in-plane brightness. In addition, it is preferable to miniaturize an illumination apparatus for converting the point light source into the plane light source as much as possible. Furthermore, it is also preferable to determine the shape of the light guiding plate and a position at which the point light source is to be disposed on the light guiding plate in light of the light usage efficiency.